There is an increasing demand for high quality premium coke for the manufacture of large graphite electrodes for use in electric arc furnaces employed in the steel industry. A number of properties are of importance in characterizing the quality of graphite electrodes. One such property is density. Usually the higher the density the better the electrode. The quality of premium coke used in graphite electrodes is also measured by its coefficient of thermal expansion, which may vary from as low as zero to as high as plus eight centimeters per centimeter per degree centigrade .times.10.sup.-7. Users of premium coke continuously seek graphite materials having higher densities and lower CTE values.
Premium coke is manufactured by delayed coking in which heavy hydrocarbon feedstocks are converted to coke and lighter hydrocarbon products. In the process the heavy hydrocarbon feedstock is heated rapidly to cracking temperature and is fed into a coke drum. The heated feed soaks in the drum in its contained heat which is sufficient to convert it into coke and cracked vapors. The cracked vapors are taken overhead and fractionated, with the fractionator bottoms being recycled to the feed if desired. The coke accumulates in the drum until the drum is filled with coke, at which time the heated feed is diverted to another coke drum while the coke is removed from the filled drum. After removal, the coke is calcined at elevated temperatures to remove volatile materials and to increase the carbon to hydrogen ratio of the coke.
In the manufacture of large graphite electrodes, calcined premium coke particles obtained from the delayed coking process are mixed with pitch, extruded to form green electrodes, and then baked at elevated temperatures to carbonize the pitch. Since pitch loses its density faster than coke the higher the percentage of coke in the mixture the greater the density of the resulting electrode. The percentage of coke can be maximized by providing the proper gradation of size of coke particles. Often in premium coking operations an excess of small particles is produced, and the resulting electrodes do not reach maximum density and strength. Providing the electrode manufacturer with coke of larger particle size gives the manufacture the flexibility to obtain desirable size distribution, e.g. by converting some large particles to particles of intermediate or smaller size. Thus it is desirable to provide a process which produces a higher proportion of larger coke particles.
Electrode performance is inversely proportional to coke CTE; reduced CTE increases electrode performance. This is reflected primarily in the lower consumption rate of electrodes with reductions in coke CTE. Thus, it is also desirable to provide a process which produces lower CTE coke.
According to this invention, premium coke having a lower CTE and increased particle size is obtained by carrying out the delayed premium coking of an aromatic mineral oil having a high aromatics content and a low molecular weight in the presence of a sparging non-coking gaseous material.